    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *     http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.configuration2.tree;
    018
    019import java.util.Collections;
    020import java.util.HashSet;
    021import java.util.Set;
    022
    023/**
    024 * <p>
    025 * A base class for node combiner implementations.
    026 * </p>
    027 * <p>
    028 * A <em>node combiner</em> is an object that knows how two hierarchical node
    029 * structures can be combined into a single one. Of course, there are many
    030 * possible ways of implementing such a combination, e.g. constructing a union,
    031 * an intersection, or an "override" structure (were nodes in the first
    032 * hierarchy take precedence over nodes in the second hierarchy). This abstract
    033 * base class only provides some helper methods and defines the common interface
    034 * for node combiners. Concrete sub classes will implement the diverse
    035 * combination algorithms.
    036 * </p>
    037 * <p>
    038 * For some concrete combiner implementations it is important to distinguish
    039 * whether a node is a single node or whether it belongs to a list structure.
    040 * Alone from the input structures, the combiner will not always be able to make
    041 * this decision. So sometimes it may be necessary for the developer to
    042 * configure the combiner and tell it, which nodes should be treated as list
    043 * nodes. For this purpose the {@code addListNode()} method exists. It
    044 * can be passed the name of a node, which should be considered a list node.
    045 * </p>
    046 *
    047 * @version $Id: NodeCombiner.java 1842194 2018-09-27 22:24:23Z ggregory $
    048 * @since 1.3
    049 */
    050public abstract class NodeCombiner
    051{
    052    /**
    053     * A default handler object for immutable nodes. This object can be used by
    054     * derived classes for dealing with nodes. However, it provides only limited
    055     * functionality; it supports only operations on child nodes, but no
    056     * references to parent nodes.
    057     */
    058    protected static final NodeHandler<ImmutableNode> HANDLER =
    059            createNodeHandler();
    060
    061    /** Stores a list with node names that are known to be list nodes. */
    062    private final Set<String> listNodes;
    063
    064    /**
    065     * Creates a new instance of {@code NodeCombiner}.
    066     */
    067    public NodeCombiner()
    068    {
    069        listNodes = new HashSet<>();
    070    }
    071
    072    /**
    073     * Adds the name of a node to the list of known list nodes. This means that
    074     * nodes with this name will never be combined.
    075     *
    076     * @param nodeName the name to be added
    077     */
    078    public void addListNode(final String nodeName)
    079    {
    080        listNodes.add(nodeName);
    081    }
    082
    083    /**
    084     * Returns a set with the names of nodes that are known to be list nodes.
    085     *
    086     * @return a set with the names of list nodes
    087     */
    088    public Set<String> getListNodes()
    089    {
    090        return Collections.unmodifiableSet(listNodes);
    091    }
    092
    093    /**
    094     * Checks if a node is a list node. This implementation tests if the given
    095     * node name is contained in the set of known list nodes. Derived classes
    096     * which use different criteria may overload this method.
    097     *
    098     * @param node the node to be tested
    099     * @return a flag whether this is a list node
    100     */
    101    public boolean isListNode(final ImmutableNode node)
    102    {
    103        return listNodes.contains(node.getNodeName());
    104    }
    105
    106    /**
    107     * Combines the hierarchies represented by the given root nodes. This method
    108     * must be defined in concrete sub classes with the implementation of a
    109     * specific combination algorithm.
    110     *
    111     * @param node1 the first root node
    112     * @param node2 the second root node
    113     * @return the root node of the resulting combined node structure
    114     */
    115    public abstract ImmutableNode combine(ImmutableNode node1,
    116            ImmutableNode node2);
    117
    118    /**
    119     * Creates a node handler object for immutable nodes which can be used by
    120     * sub classes to perform advanced operations on nodes.
    121     *
    122     * @return the node handler implementation
    123     */
    124    private static NodeHandler<ImmutableNode> createNodeHandler()
    125    {
    126        return new AbstractImmutableNodeHandler()
    127        {
    128            @Override
    129            public ImmutableNode getParent(final ImmutableNode node)
    130            {
    131                return null;
    132            }
    133
    134            @Override
    135            public ImmutableNode getRootNode()
    136            {
    137                return null;
    138            }
    139        };
    140    }
    141}